For the latest updates on the Mendocino Complex Fire, read this story.

A fire that has been growing for nearly two weeks in Northern California, becoming the largest in modern state history, continued to rage overnight, as firefighters battled to keep the edges from eating into residential areas, officials said on Tuesday.

The Mendocino Complex Fire, which is burning northwest of Sacramento, reached more than 290,600 acres as of Tuesday morning, said Cal Fire, the state’s fire agency. It overtook last year’s 282,000-acre Thomas Fire on Monday to become the most sizable California fire in a century of record-keeping.

Benjamin Nicholls, a division chief of Cal Fire, said on Tuesday morning that crews were battling the expanding blaze along the northern edges of the fire, where it extends into mostly forested areas. He said crews were working on hot spots to make sure the fire does not creep into residential areas in the south.

“The increase of acreage is into the forest,” he said in an interview. “The fire is holding on the south side.”

Lynne Tolmachoff, a spokeswoman for the agency, said on Monday that the state was in only the middle of its fire season, noting that the worst fires often occur later in the year, as the land becomes increasingly dry and weather patterns create windy conditions.

Larry Dietz, a spokesman for the Red Cross, said there were about 500 people in six Red Cross shelters and in one other shelter that is not affiliated with the agency. Residents are given bedding, kits with toiletries and meals.

“It is kind of smoky here,” he said, speaking by telephone from Middletown, which has a population of about 1,300 people. ‘We have a very high proportion of people that are in the shelter requiring significant medical care, such as mobility issues and use of oxygen tanks. It is my understanding that they had to evacuate a skilled nursing facility.”

The Mendocino Complex Fire is a combination of two fires that ignited a few miles apart, Ms. Tolmachoff said. In instances where multiple blazes are close enough together and affect the same area, she said, officials consider them one fire, called a complex.

Officials are investigating the cause of the fire, which started on July 27 and was 34 percent contained as of early Tuesday, Cal Fire said in its latest update.

It has destroyed 143 structures as of Tuesday: 75 residences and 68 other types of structures.

Despite its size, no one has died in the Mendocino Complex Fire, Ms. Tolmachoff said. By contrast, the Carr Fire, which is also burning in Northern California, has killed seven people and destroyed more than 1,600 buildings. It is the 12th largest in California history, at about 164,000 acres.